The 21cm "Outer Arm" and the Outer-Galaxy High-Velocity Clouds: Connected by Kinematics, Metallicity, and Distance

TL;DR

This study uses ultraviolet spectra to analyze the metallicity, kinematics, and distances of the Outer Arm and high-velocity clouds in the outer Galaxy, revealing potential common origins and complex gas properties.

Contribution

It provides new measurements of metallicity, distance, and kinematic connections between the Outer Arm and high-velocity clouds using high-resolution UV spectroscopy.

Findings

Outer Arm detected in absorption toward two QSOs

Galactocentric radius of Outer Arm constrained to 9-18 kpc

Outer Arm metallicity estimated at 0.2-0.5 Z_{solar}

Abstract

Using high-resolution ultraviolet spectra obtained with the HST/Space Telescope Imaging Spectrograph (STIS) and the Far Ultraviolet Spectroscopic Explorer, we study the metallicity, kinematics, and distance of the gaseous "Outer Arm" (OA) and the high-velocity clouds (HVCs) in the outer Galaxy. We detect the OA in a variety of absorption lines toward two QSOs, H1821+643 and HS0624+6907. We search for OA absorption toward eight Galactic stars and detect it in one case, which constrains the OA Galactocentric radius to 9<R_{G}<18 kpc. We also detect HVC Complex G, which is projected near the OA at a similar velocity, in absorption toward two stars; Complex G is therefore in the same region at R_{G} = 8 - 10 kpc. HVC Complex C is known to be at a similar Galactocentric radius. Toward H1821+643, the low-ionization absorption lines are composed of multiple narrow components, indicating the presence of several cold clouds and rapid cooling and fragmentation. Some of the highly ionized gas is also surprisingly cool. Accounting for ionization corrections, we find that the OA metallicity is Z=0.2-0.5 Z_{solar}, but nitrogen is underabundant and some species are possibly mildly depleted by dust. The similarity of the OA metallicity, Galactocentric location, and kinematics to those of the adjacent outer-Galaxy HVCs, including high velocities that are not consistent with Galactic rotation, suggests that the OA and outer-Galaxy HVCs could have a common origin.